This invention relates to driver circuits for lasers suitable for use in a laser printing system and, more particularly, to the use of dual feedback loops for controlling current pulses applied to a laser diode, the invention including circuitry for isolating the driver form changes in the amplitude of a digital input signal and for introducing a threshold current to a laser drive signal.
The use of a laser, in the form of a diode, as the source of radiation in a laser printing system has found wide acceptance in printing systems, including laser printers used as an output device of a computer and in facsimile machines. Typically, the radiation, or light, of the laser is scanned across a photosensitive material which serves as a recording medium for recording data. The scanning of the light across the photosensitive material is accomplished, in some equipment, by a moving optical element providing an optical scan of the beam. in other equipment, the scanning is accomplished by introducing relative motion between the photosensitive material and a print heat incorporating the laser. It is also common practice to include both optical and mechanical scanning of the laser beam in a printing apparatus.
Of particular concern herein is the use of a laser diode for producing a digitally modulated laser beam suitable for use in a printing system wherein the image on the photosensitive medium is composed of an array of light and dark spots which appear to the human eye as alphanumeric symbols or gray-scale pictorial data. Typically, the imprinted data to such a print system is in the form of a digital command signal provided by a character generator or other source of data, the command signal designating the intervals of time over which the laser is turned on at sufficient intensity to place a mark on the recording medium and those intervals of time when the intense radiation of the laser is to be extinguished or to be reduced to a sufficiently low intensity having essentially no effect upon the recording medium.
To optimize the operation of such a print system, it is desirable to sense the actual value of current flowing through the laser diode in response to the digital command signal so as to ensure an accurate production of laser radiation, or light, of the desired intensity. This assures uniformity in creating an image on the recording medium. A further improvement in the uniformity of the image is attained by sensing the intensity of light produced by the laser, this being useful to counteract any effect of drift in the laser characteristics of light output versus drive current. Yet a further consideration in the operation of the laser diode concerns the extent to which the laser drive current is to be terminated between pulses of radiation for facilitating the activating and deactivating of the laser. It is also important to operate the laser in a manner which enhances high-speed production of light pulses because a high-speed pulse train can increase the throughput rate of the print system, or alternatively, can provide a higher resolution image with more pixels per inch at a given throughput rate.
Consequently, it is an object of this invention to provide a laser drive which optimizes the use of all of the foregoing parameters for enhancing image quality, both in terms of image intensity and image resolution.